This invention relates to preamplifiers for powered transducers such as condenser microphones, and in particular to such preamplifiers that directly couple the preamplifier input with the preamplifier output.
As is well known in the art, powered transducers such as condenser microphones are biased with a phantom voltage such as +48 VDC. It is customary to use a reactive component such as a capacitor or a transformer to isolate the preamplifier from the biasing voltage. Such reactive components can introduce a phase shift known as signal group delay which often varies as a function of frequency, thereby producing distortion.
The present invention is directed to an improved preamplifier for a powered transducer that substantially overcomes these drawbacks of the prior art.
The powered transducer preamplifiers described below include a preamplifier circuit having a preamplifier input, a preamplifier output and a preamplifier signal path that DC couples the input and the output. A biasing circuit is coupled to the input to apply a DC bias voltage to the input, thereby powering the transducer, and a DC level shifting circuit is DC coupled to the signal path between the input and the output. This DC level shifting circuit compensates for the bias voltage by substantially or completely eliminating the bias voltage at the output.
Because the entire signal path between the preamplifier input and output is DC coupled, phase distortion associated with the capacitive and inductive components used in the prior art described above are completely eliminated.
A wide range of circuits can be used to implement the preamplifier circuit, the biasing circuit, and the DC level shifting circuit, as described in detail below. This section has been provided by way of introduction, and it is not intended to limit the scope of the following claims.